The rice field biofertilizer Azolla microphylla was subjected to salinity treatment (EC 4.70 and 8.72 ds/m) and growth, nitrogen fixation and the activity of antioxidant enzymes such as superoxide dismutase, ascorbate peroxidase and catalase were studied. Salinity exposure resulted in decreased growth rate and nitrogen fixation by the organism. Further a positive correlation between the peroxide content, and antioxidant enzyme activity in relation to the level of salinity was also observed. From the results we conclude that the observed changes in the antioxidant enzyme profile of A. microphylla could be effectively used to bio-monitor the toxic effect due to salinity. An increase in the activity of antioxidant enzymes could be therefore exploited further to develop biosensors to detect harmful free radicals and the extent of cell damage in the cells. (C) 2010 Elsevier B.V. All rights reserved.